1. Field of the Invention
The present invention relates generally to laser ablation, and more particularly pertains to a method and system for enhancing the quality of laser ablation by controlling the laser repetition rate.
When using femtosecond lasers to ablate materials used in photomasks, such as Molybdenum Silicide (MoSi), the material, and therefore the photomask, can be irreversibly damaged by the ablation process. This damage can occur extending across the entire ablated region, resulting in damaged material that cannot be subsequently ablated. Even with implementing a careful control of the ablation process, the edges of the ablation region are usually damaged.
In the process of removing defects from photomasks, the damage caused by the ablation process prevents iteratively trimming the edges of the region which are to be repaired. This limits the spatial resolution of the ablation process, and the ability of tool operators to accurately position a repair. In addition, the ablation process often results in the redepositing of material around the repaired region. This redeposited material can appear in the form of large particles, particularly at the edges of the repaired region. When material is redeposited in this manner, this may result in a “new” mask defect, whereby this new mask defect is difficult to remove.
2. Discussion of the Prior Art
The foregoing issues have been addressed to some extent in the technology, with varying degrees of success, as set forth in the hereinbelow listed prior art publications. However, none of the publications have fully met the requisite intent of the present invention. These publications are as follows: Grenon, et al., U.S. Pat. Nos. 6,190,836; 6,165,649; 6,156,461; 6,090,507; and Haight, et al., U.S. Pat. No. 6,333,485.
This aspect is also discussed in various further publications, such as, for instance, the following articles: “Laser Induced Periodic Surface Structure: Experiments on Ge, Si, Al, and Brass”, Young, Preston, vsn Driel, and Sipe, Physical Review B, Vol. 27, No. 2, pgs. 1155-1172 (1983); “Ultraviolet Laser Induced Periodic Surface Structures”, Clark and Emmony, Physical Review B, Vol. 40, No. 4, pgs 2031-2041 (1989); “Femtosecond Laser Induced Periodic Surface Structure on Diamond Film”, Wu, Ma, Fang, Liao, Yu, Chen, Wang, Applied Physics Letters, Vol. 82, No. 11, pgs 1703-1705 (2003); and “Self Organized Nanogratings in Glass Irradiated by Ultrashort Light Pulses”, Shimotsuma, Kazansky, Qui, Hirao, Physical Review Letters, Vol. 19, No. 24, pgs 247205-1 to 4 (2003).